Commander Keen is a series of side-scrolling platform video games developed primarily by id Software. The series consists of six main episodes, a "lost" episode, and a final game; all but the final game were originally released for MS-DOS in 1990 and 1991, while the 2001 Commander Keen was released for the Game Boy Color. The series follows the eponymous Commander Keen, the secret identity of the eight-year-old genius Billy Blaze, as he defends the Earth and the galaxy from alien threats with his homemade spaceship, rayguns, and pogo stick. The first three episodes were developed by Ideas from the Deep, the precursor to id, and published by Apogee Software as the shareware title Commander Keen in Invasion of the Vorticons; the "lost" episode 3. 5 Commander Keen in Keen Dreams was developed by id and published as a retail title by Softdisk; episodes four and five were released by Apogee as the shareware Commander Keen in Goodbye, Galaxy; and the simultaneously developed episode six was published in retail by FormGen as Commander Keen in Aliens Ate My Babysitter. Ten years later, a homage and sequel to the series was developed by David A. Palmer Productions and published by Activision as Commander Keen.